The termination of El Niño events

Understanding the processes that control the termination of El Niño is essential to representing them properly in dynamical prediction schemes, and to understand how the duration of events may change in changing climate. We explore the processes that lead to the end of El Niño events using a combination of data analysis, and forced ocean general circulation model (OGCM) and coupled general circulation model (CGCM) experiments. We find that meridional changes the western Pacific zonal wind anomaly field are fundamental to the termination of El Niño events, and representing these processes is important in forecasts of the termination of El Niño.

We find that the termination of recent El Niño events has been characterized by (1) a late-year southward shift of near-Dateline westerly wind anomalies, and (2) subsequent cold tongue thermocline shallowing is driven by the wind shift. OGCM experiments establish the shift as the primary cause of cold tongue thermocline shallowing. Reflected equatorial waves and local wind anomaly changes are much less important in setting the timing. The structure and timing of the meridional changes suggests that interactions between the seasonal cycle of solar heating and anomalous El Niño conditions lead to the termination.

Analysis of the GFDL CGCM shows that the late-year southward shift of zonal wind anomalies is also a characteristic feature of the termination of El Niño in the coupled model. This southward shift is followed by thermocline shallowing and a cooling of eastern equatorial Pacific SSTs. We perform perturbation experiments with the CGCM to determine the processes that drive the meridional changes in the zonal wind anomaly field, and find that interactions between the seasonal cycle of solar insolation and anomalous El Niño conditions are a dominant mechanism for the termination of El Niño events.

Succesful theories and models of El Niño should either represent or parametrize the processes that cause the meridional changes in the west Pacific wind anomaly field, since these processes are fundamental to the termination of El Niño.